Mt Tongariro in the central North Island has erupted, GNS Science says.

An eruption and a plume of smoke from the Te Maari Crater, on the northern side of Tongariro, was captured on web camera about 1.30pm on Wednesday.

The eruption prompted the volcanic alert level was raised to "minor eruptive activity" and the aviation colour code to red - "eruption is forecast to be imminent and significant ash emission into the atmosphere likely".

"There was one eruption, essentially one explosion, and it was not sustained. It spewed some ash possibly a couple of kilometres up the air. The ash is now drifting to the east-southeast.''

Adrift NZ, which runs tours of the Tongariro Alpine Crossing, had around 50 people on the mountain today.

Operator Stewart Barclay, who chairs a group of 30 users of Mt Tongariro, was heading to Mt Tongariro to help his guides and their groups off the mountain.

"I'm just going to make sure everyone is safe... from my guides perspective and from what I've heard, it seems minor."

Mr Barclay understood the eruption had launched no projectiles into the air.

"I've just talked to one of my guides and he is taking the safe route out. There was a minor amount of panic and everyone is safe now, there were no injuries."

Dr Fournier said the eruption was not very loud and was smaller than an earlier eruption in August.

Dr Fournier was a few kilometres away when the eruption happened and said while he was safe, he had "a good view of the explosion''.

"It was remarkable a few minutes ago, now it's dying off,'' he said about 20 minutes after the explosion.

"When it happened you had a column of the ash which spewed in the air vertically, and then essentially the source of that eruption just stopped.''

Dr Fournier said there was "quite a bit of gas'' but it was quietening down and the ash was drifting away.

The next step was to work with the Department of Conservation (DoC) and authorities to make sure people in the area were safe.

"That's going to be the top thing, so right now we are actually with DoC and liaising with the authorities just to make sure that everybody is aware that there was an eruption, how big it was and where the ash is going.

"Then the authorities will take it from there, if there is any action to be made.

"But at the moment it's probably much less of a big eruption than it was in early August.''

GNS Science has increased the aviation colour code from yellow to red following the eruption. The red status means an eruption "is forecast to be imminent with significant emission of ash into the atmosphere likely".

A police spokesperson said the Desert Road remains open.

Tongariro, in the centre of the North Island, erupted in August for the first time in 115 years, sending ash as far east as Napier.

According to GeoNet, Tongariro is a complex of multiple volcanic cones constructed over a period of 275,000 years.

The mountain's active vents include Te Maari, Emerald, North Crater and Red Crater.

Last week GNS Science increased the likelihood of neighbouring volcano Mt Ruapehu erupting, following increased activity on the mountain.

The eruption was captured on the Te Maari Crater camera at around 1.25pm this afternoon.

An operation is underway to get people off the Tongariro Crossing.

Volcanologists have advised that the Volcanic Alert is now Level 2.

Do you have photos or video? Send them to news@tvnz.co.nz

Brent Guy from the Department of Conservation told ONE News that the eruption was small.

He said it appears to be a gas eruption.

The Aviation Colour Code is now red, meaning that a significant amount of gas has been emmitted into the atmosphere.

Civil Defence has advised people to visit civildefence.govt.nz or follow @NZcivildefence on Twitter for Ministry of Civil Defence information on the eruption.

They have also told people living in or near the Waikato, Hawke’s Bay, Gisborne and Bay of Plenty areas to stay indoors.

Meanwhile, a bus company has confirmed that around 100 teachers and children from Tamatea Intermediate were about two hours into a tramp on the Tongariro track. Bus drivers are waiting for children to come out.

And a staff member from the Skotel, a hotel on Mount Ruapehu said 6 to 8 guests are on the mountain.

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GNS Science confirmed the eruption, at the Te Maari crater, happened shortly after 1.20pm. It is the second eruption on the mountain this year; an eruption on August 6 was the first on Tongariro for more than 100 years.

Up to 90 Napier schoolchildren were on the mountain, two hours into a tramp on the Tongariro track, but are reportedly safe and well.

Tamatea Intermediate teacher Lomi Schaumkel said the group - which also included six parents and four teachers - was near the Katetahi hot springs when they saw the beginnings of the eruption.

''We were right up there next to it. It was just amazing. We were probably only a kilometre away from it. We were right next to one of the signs saying we were out of the danger zone.

''We saw all these tourists running away from it. We didn't stick around long,'' he said.

A further 20 Year 8 pupils from Gulf Harbour School in Whangaparoa were making their way off the track with parents and guides after being just 750m from where the crater erupted.

Two bus drivers from Nimon and Sons, who took the Napier children to the mountain, had reported back to their base that they could see an ash plume 2km high, a spokesman said.

Conservation Department (DOC) area manager Jonathan Maxwell said 30 to 50 people were being evacuated from the Tongariro Crossing track. No injuries had been reported. State highways in the area had been closed.

Today's eruption lasted for about five minutes. GNS has updated the alert on Tongariro to level 2, meaning there is "minor eruptive activity". The aviation colour code has been lifted to red, meaning there is "significant emission of ash'' into the atmosphere.

Air New Zealand flights between Taupo and Wellington have been cancelled because of the volcanic activity.

An airline spokesman said the company was working with the Civil Aviation Authority and the MetService to monitor the ash cloud.

Passengers were being advised to check the Air New Zealand website for flight arrivals and departure information which would be constantly updated throughout the day.

A national advisory has been issued by Civil Defence saying regions from Waikato down to Hawke's Bay could be affected by ash cloud.

"Minor volcanic activity is occurring at Tongariro - Te Maari craters and could be hazardous in the immediate vicinity of the craters.

"Light volcanic ash fall can be anticipated downwind of Tongariro and may impact the following areas or regions: Waikato; Hawkes Bay; Gisborne and Bay of Plenty."

Genesis Energy was shutting production at its Tongariro hydro scheme site south of Lake Taupo.

Tongariro/Rangipo Prison - which is located near the base of Mt Tongariro, off the Desert Road - was operating as normal at this stage.

Corrections Department central regional manager Terry Buffery said there was no local site impact following the eruption.

''However, robust plans are in place to manage the facility should the situation escalate. Safety is always a priority for staff, prisoners and the public. We are confident in our ability to respond accordingly.''

STAY INDOORS

Civil Defence has warned people to stay indoors if ash is present.

"Volcanic ash could be a health hazard, especially if you suffer from breathing difficulty. If outside, seek shelter (e.g. in a car or building)."

It was advising people to close all windows and cover their mouths if caught in the ash. People should stay out of designated restricted zones.

DOC had a helicopter in the air and was canvassing the volcano to see if they could locate anyone on the mountain, a Taupo Council spokeswoman said.

"There is a helicopter looking to see if there's anyone around, any visitors on the mountain to make sure everybody is safe," she said.

Taupo Council's civil defence emergency manager Phil Parker, who coordinated the response last time the volcano erupted, was working with DOC and other organisations.

RESIDENTS ALARMED

Lake Rotoaira resident Robyn Bennett said there was a big, black ash cloud over her house, which was about a kilometre from the eruption site.

"It's just blew her stack," she said.

She said the air smelled of sulphur.

"It's hard to breathe if you go outside, it's pushing out quite heavily." Bennett said she didn't hear the eruption but it looked like a new vent had formed in front of a previous eruption crater. The ash cloud was moving east towards Napier and Taupo.

Ann Lambert, owner of the Rainbow Motel at Tokaanu, near Turangi, said she had not heard any noise, unlike the previous time Tongariro erupted when there had been a "huge loud noise".

"We weren't aware it had happened. We just looked up and saw it," she said.

"It's just a grey cloud at the moment ... It's not as imposing as the last time."

Kathleen Konui, who lives at Otukou Marae "just below" the mountain, said she had heard a sound like a shotgun going off.

"The mountain was all covered with white smoke," she said.

In the hour or so since then the air had cleared, although she could still see some smoke coming from about four holes on the mountain.

"I'm standing here now looking at them."

The sky was too hazy for her to be able to see a plume from the eruption.

At 1.30am this morning a New Zealand Couriers truck driver reported a strong smell of sulphur on the Desert Road while Motuoapoa resident Cindy Greaney said she noticed an "obvious smell of sulphur" at 6am.

HISTORY REPEATS

This is the second eruption this year. The Te Maari crater erupted on August 6, the first time in more than 100 years.

The eruption, on August 6, widened and deepened the crater and reactivated vents which had been covered up in the 116 years since it last erupted in 1896.

Ruapehu has also been active recently but has not erupted. Pressure was building under the volcano and there was an increased likelihood of an eruption, GNS said last week.

The volcanoes are close to each other but scientists did not believe that the activity at both volcanoes was related.

However, they couldn't exclude the possibility, GNS vulcanologist Nico Fournier said earlier this week.

"We don't have evidence at the moment that the activity of the two volcanoes are related," Fournier said. "But we can't exclude it entirely."

GNS vulcanologists were researching if activity at the two volcanoes was connected and also were looking at the relationship between earthquakes and volcanic activity.

A series of earthquakes rattled the ground beneath Tongariro in the weeks leading up to the August eruption and another series of quakes have shaken the ground beneath Ruapehu in the last few weeks.

* Airports are all open. Air New Zealand has issued a warning that flights might be disrupted and urged passengers to check its website for any delays or cancellations. Two flights between Wellington and Taupo have been cancelled. Wellington Airport spokesman Greg Thomas said the situation was being monitored.

* Genesis Energy is shutting production at its Tongariro hydro scheme site south of Lake Taupo.

* State highways in the Tongariro area have been closed.

* Tongariro/Rangipo Prison, located near the base of Mt Tongariro, was operating as normal.

* A contingency plan has been put in place for the Lake Taupo Cycle Challenge, to be held at the weekend.

* Civil Defence has issued a national advisory for regions from Waikato down to Hawke's Bay, which could be affected by ash cloud.

Mt Tongariro erupted today just before 1.30pm, its second bout of volanic activity this year. Here is a wrap-up of the mountain's history.

August 6, 2012: The volcano erupts for the first time in more than 100 years. It was a gas-driven eruption and created new vents

1869: A large eruption (accompanied by an earthquake) formed the upper Te Maari Crater during an explosive eruption. M&#257;ori descriptions include a "bright red flame through the smoke that would burst and fall like snow".

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